In some wireless communication systems, the frequency bandwidth of a transmission from a first wireless communication device to a second wireless communication device is divided into several subchannels. For example, in a WiMAX™ wireless communication system, in the adaptive modulation and code (AMC) mode, there are multiple subchannels in a transmission frame and each subchannel comprises a plurality of subcarriers. More specifically, there are 24 subchannels in the WiMAX AMC mode, and the bandwidth of one subchannel comprises 18 subcarriers that span approximately 200 kHz. In the AMC mode, a downlink (DL) transmission from a base station to a mobile station or an uplink (UL) transmission from a mobile station to the base station occupies one or more of these subchannels.
The wireless link between two wireless communication devices can be time-varying. Consequently, the link quality in each subchannel may change over time. The link quality can be so bad at certain times that the subchannel can not be used to reliable carry a message even with maximum transmit power and a low data rate. At these times, it is better practice not to allocate data (energy) to that subchannel by effectively “disabling” the subchannel for a period of time.
In some wireless communication systems, such as a WiMAX system, the channel bandwidth is generally larger than the so-called coherent bandwidth. As a result, the correlation between different subchannels may be low. Therefore, the subchannels may vary differently over time and also independently experience different types of link quality variations.
In a TDD WiMAX system, the beamforming weight vector that the base station uses for each DL subchannel to a particular mobile station is estimated from a transmission that the base station receives from the particular mobile station on the corresponding UL subchannel. Therefore, when one UL subchannel is disabled, extra effort is needed at the base station to compute beamforming weights for the corresponding DL subchannel.
Accordingly, when applying a channel disable scheme to a wireless channel that behaves in this way, it is desirable to apply a disable scheme on each subchannel separately and independently. Moreover, a mechanism is needed to compute beamforming weights used by the base station for a DL transmission to a particular mobile station when one or more subchannels in an UL transmission from the particular mobile station are disabled.